Tunable circuit for tunable capacitor devices

ABSTRACT

A tunable circuit ( 10 ) for a capacitively tunable capacitor device ( 12 ) is provided. The tunable circuit ( 10 ) comprises a tunable circuit element ( 14 ) and a non-tunable dielectric element ( 16 ) coupled to the tunable circuit element ( 16 ). A tunable capacitor device ( 12 ) and a method for increasing the figure of merit in a tunable capacitor device ( 12 ) are also provided.

CONTRACTURAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC36-99GO-10337 between the U.S. Department of Energy and theNational Renewable Energy Laboratory, a Division of Midwest ResearchInstitute.

TECHNICAL FIELD

This invention relates generally to a tunable circuit for use in RFtunable devices where the tuning is achieved via variable capacitanceeither in a lumped element capacitor or in distributed circuits and,more particularly, it relates to a tunable circuit which increases-thefigure of merit (performance vs. noise) and achieves the low voltagerequirements for a practical tunable capacitor device by coupling a lowloss, non-tunable capacitive element with a tunable element.

BACKGROUND ART

Tunable RF devices such as filters, phase shifters, and oscillators aretypically built using semiconductor diodes, so called varactors, inwhich the capacitance is controlled via external bias. While the mainline varactors are inexpensive and robust, they are only suitable forapplications up to 10 GHz. Above this frequency, the energy dissipatedin such varactors is prohibitively high (low quality factor Q). In someGaAs varactors, the range of operation is extended to much higherfrequencies. The high cost of manufacturing for such devices, however,makes them impractical for most applications.

Recently, tunable dielectrics, such as Balium Strontium Titanate (BST),have been employed as the active elements in tunable capacitor devicesand are becoming increasingly important for a large number of microwaveapplications. Utilizing a tunable dielectric element in tunablecapacitance devices, especially at frequencies over 20 GHz, has beenshown to increase the figure of merit (performance vs. noise) of thetunable capacitor device with a lower cost than other conventionaltechnologies. BST thin film and especially BST/MgO thick and thin filmscomposites have demonstrated unparalleled performance at high MWfrequencies up to 60 GHz. They also have low power requirements, butneed voltages in some applications. Thus, incorporation of the tunabledielectric elements provides high performance at low cost.

While the figure of merit of the tunable dielectric devices can besufficiently high, such as those with composite materials, the voltagerequirements of these devices are typically too high (300V). Thestandard employed for the lower frequency applications typically designsfor tuning voltages in the range of 20–40 V. There is a pressing need todevelop lower voltage tunable devices with a high figure of merit so asto achieve high levels of performance at microwave frequencies, i.e.,this requires the amount of tuning to be maximized and the amount ofloss to be minimized, while satisfying industry requirements for the lowoperating voltages.

DISCLOSURE OF THE INVENTION

The present invention is a tunable circuit for capacitively tunabledevices. The tunable circuit comprises a tunable circuit element and anon-tunable dielectric element coupled to the tunable circuit element.At least one AC terminal contacts the non-tunable dielectric element.

The present invention additionally includes a method for substantiallyincreasing the figure of merit in a tunable capacitor device. The methodcomprises providing a tunable element, providing a non-tunable element,and coupling the tunable element to the non-tunable element.

The present invention further includes a tunable capacitor device. Thetunable capacitor device comprises a non-tunable dielectric element anda tunable dielectric element. The tunable dielectric element iselectrically connected to the non-tunable element thereby forming acombined dielectric element. A plurality of contacts are mounted to thecombined dielectric element with at least one of the contactselectrically connected to the non-tunable dielectric element.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the preferred embodiments of the presentinvention, and together with the descriptions serve to explain theprinciples of the invention.

In the Drawings:

FIG. 1 is a circuit diagram of the tunable dielectric circuit,constructed in accordance with the present invention, with a non-tunableelement coupled together with a tunable element;

FIG. 2 is an elevational side view of an embodiment of the circuitdiagram in FIG. 1 for the tunable dielectric circuit, constructed inaccordance with the present invention, with the non-tunable elementcoupled together with the tunable element in a layered structure;

FIG. 3 is another circuit diagram for the tunable dielectric circuit,constructed in accordance with the present invention, with athree-electrode or four-electrode configuration allowing retention ofthe low control voltages of the combined tunable element.

FIG. 4 is an elevational side view of the embodiment of the tunabledielectric circuit as in FIG. 3, constructed in accordance with thepresent invention, with the three-electrode or four-electrodeconfiguration in a layered structure;

FIG. 5 is another embodiment of the two terminal tunable circuit,constructed in accordance with the present invention; where thenon-tunable and tunable lumped element capacitors are combined togetheras in the circuit diagram in FIG. 1.

FIG. 6 is another embodiment of the more than two terminal tunablecircuits, constructed in accordance with the present invention, wherethe tunable and non-tunable lumped element capacitors are combinedtogether as in the circuit diagram of FIG. 3 with the three terminalconfiguration;

FIG. 7 is a perspective view of another embodiment of the tunablecircuit diagram with four terminals, constructed in accordance with thepresent invention, with the low loss dielectric substrate such as LaAlO₃or MgO or another dielectric providing mechanical support for thetunable dielectric thin film and also serving as non-tunable dielectricelement electrically coupled to the tunable dielectric and the bottomelectrodes being connected to the substrate are the AC terminals whilethe top electrodes being directly attached to the tunable dielectric arefor the DC voltage control; and

FIG. 8 is a perspective view of another embodiment of the tunabledistributed circuit (coplanar waveguide phase shifter) where the lowloss non-tunable dielectric layer is included to improve the performanceof the device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in FIGS. 1–8, the present invention is a tunable highfrequency circuit, indicated generally at 10, for use in a capacitivelytunable device 12. The tunable device 12 can contain any type of tunablecapacitor where the figure of merit is limited by the loss including,but not limited to, semiconductor varactors, tunable dielectriccapacitors, and distributed elements with adjustable capacitance such asmight be used in electronically steerable antennas, oscillators,filters, and phase shifters.

The present invention relates to lumped element tunable capacitors suchas semiconductor varactors, tunable dielectric capacitors, and any othertunable capacitive elements limited by loss performance. It also relatesto distributed circuits such as, for example, coplanar phase shifterswhere the tuning action is achieved by changing the dielectric constantof a tunable dielectric media (changing equivalent capacitance) with DCbias.

As illustrated in FIG. 1, in an embodiment of the tunable circuit 10,the non-tunable dielectric element 16 can be coupled together with thetunable circuit element 14 as a single lumped element. As illustrated inFIG. 2, in another embodiment of the tunable dielectric circuit 10, thenon-tunable dielectric element 16 can be coupled together with thetunable circuit element 14 in a layered structure. In FIG. 1, C1represents the tunable circuit element 14 and C2 represents thenon-tunable dielectric element 16. AC represents the microwave signal 18and DC represents the bias voltage 20. The improved tunable circuit 10of FIGS. 1 and 2 provides improved figure of merit (tuning/loss)parameters in tunable circuit elements 14 and in lumped elements withnon-tunable dielectric elements 16 where the improvement occurs becauseof the ability to improve the Q factor for the tunable capacitor device12. The bias voltage in this configuration increases compared to thebias voltage of the tunable element alone.

Still referring to FIG. 2, to construct the tunable dielectric circuit10 of the present invention, the tunable dielectric element 14 is formedon a substrate 22. The non-tunable dielectric element 16 is then layeredonto the tunable dielectric element 14. Next, a pair of contacts 24 iselectrically connected to the non-tunable dielectric element 16. Whilethis embodiment of the tunable dielectric circuit 10 of the presentinvention results in substantial improvement in the figure of merit ofthe tunable capacitor device 12, higher potentials or voltage arerequired due to the potential of the tunable capacitor device 12extending across both the tunable circuit element 14 and the non-tunabledielectric element 16.

As illustrated in FIG. 3, in still another embodiment, the improvedtunable dielectric circuit 10 of the present invention also includes the“three- or four-electrode” design that allows the improvement of thetuning/loss ratio of a tunable circuit element 14 without increasingcontrol voltages. As illustrated in FIG. 4, in yet another embodiment ofthe tunable dielectric circuit 10 of the present invention, a layeredstructure positions the DC bias 20 only across the tunable dielectricelement 14 but extracts the AC signal 18 from the whole tunablecapacitor device 12.

Still referring to FIG. 4, to construct the tunable dielectric circuit10 of the present invention, the non-tunable dielectric element 16 isformed on the substrate 22. The tunable circuit element 14 is thenlayered onto the non-tunable dielectric element 16. Next, a pair ofcontacts 24 is electrically connected to the tunable dielectric element14 and a contact 24 is electrically connected to the non-tunabledielectric element. The embodiments of the tunable dielectric circuit 10of the present invention, as illustrated in FIGS. 3 and 4, have thefurther advantage of increasing the figure of merit of the tunablecapacitor device 12 while maintaining the low voltage requirement of thetunable dielectric element 14.

As illustrated in FIG. 5, another embodiment of the present invention,the non-tunable dielectric lumped element capacitor 16 is coupled withthe tunable circuit lumped element capacitor 14 to improve the figure ofmerit of the tunable capacitor device 12. The tunable dielectric circuit10 is low cost and potentially can be integrated with most designs oftunable capacitor devices 12.

As illustrated in FIG. 6, another embodiment of the present invention,the non-tunable dielectric lumped element capacitor 16 is coupled withthe tunable circuit lumped element capacitor 14 to improve the figure ofmerit of the tunable capacitor device 12. The third terminal addedbetween the two capacitors will allow maintaining low control voltage ofthe tunable element 14. The tunable dielectric circuit 10 is low costand potentially can be integrated with most designs of tunable capacitordevices 12.

As illustrated in FIG. 7, another embodiment of the present invention isshown. In this arrangement, the low loss dielectric substrate, such asLaAlO₃ or MgO or another dielectric provides mechanical support for thetunable dielectric thin film and also serves as a non-tunable dielectricelement electrically coupled to the tunable dielectric circuit layer 14.Furthermore, in this arrangement, the bottom electrodes 24 connected tothe substrate are the AC terminals while the top electrodes 24 aredirectly attached to the tunable dielectric are for the DC voltagecontrol.

As illustrated in FIG. 8, another embodiment of the present invention, alayer of non-tunable dielectric element 16 coupled in series withtunable dielectric circuit 14 and the electrodes 24 of the waveguidecontacting the non-tunable dielectric element 16.

In constructing the non-tunable dielectric element 16, non-tunablematerials such as an inorganic solid-state dielectric material ordielectric polymer can be used. As illustrated in FIG. 6, the dielectricpolymer is shown. The polymer non-tunable dielectric element 16 can bedeposited by physical vapor deposition, spin coated, or ink jet writtenor deposited by other means on the tunable capacitor device 12significantly improving fabrication of the tunable capacitor circuit 10.It is also within the scope of the present invention to utilize variouspolymers or polymer mixes to adjust the dielectric constant of thenon-tunable dielectric element 16 so as to optimize performance of thetunable dielectric circuit 10.

A wide variety of polymers are available for a wide range of dielectricconstants and processing temperatures for constructing the non-tunabledielectric element 16. Dielectric constants can be easily adjusted fromtwo (2) to eight (8), for example. In addition, polymers with goodbreakdown characteristics may be chosen. For polystyrenes, for example,dielectric strengths are in the range of 100–600 KV/cm while in thePolyethylene terephthalate, the dielectric strengths can be up to 6000KV/cm. The incorporation of polymers as the non-tunable dielectricelement 16 reduces cost, improves design flexibility, and improves theease of fabrication.

As discussed above, the essence of the present invention is to increasethe figure of merit of the tunable capacitor devices 12, i.e., improvetuning and reducing loss. By coupling the microwave signal into anon-tunable low loss capacitance element 16 in series with aconventional tunable capacitive element 14, the figure of merit of thetunable capacitor device 12 is improved.

The tunable capacitor circuit 10 of the present invention is thesolution for the tunable capacitor devices 12 when the limit of theperformance is set by a low tuning/loss ratio (particularly for highloss situations) of a tuning element as is usually the case for thesemiconductor and ferroelectric based tuning elements in the microwavefrequency range, especially above ten (10) GHz. For the existingsemiconductor and ferroelectric based tuning elements, several fold (atleast 2–5 times) improvement in tuning/loss parameter is possible.Additional benefits are the improved power handling capability of tuningelements 14 (especially an issue for semiconductors) and reduced tuningvoltages and improved temperature stability for the ferroelectric tuningelements. The improvement in the figure of merit as in the presentinvention will be realized at any RF frequency and any temperature aslong as the loss of the non-tunable component is significantly lowerthan that of the tunable component. The potential embodiments of thetunable capacitor circuit 10 of the present invention include multilayerintegrated structures combining high loss tunable and low lossnon-tunable layers or components, or separate lumped element capacitorsintegrated into a circuit in series.

The foregoing exemplary descriptions and the illustrative preferredembodiments of the present invention have been explained in the drawingsand described in detail, with varying modifications and alternativeembodiments being taught. While the invention has been so shown,described and illustrated, it should be understood by those skilled inthe art that equivalent changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention, andthat the scope of the present invention is to be limited only to theclaims except as precluded by the prior art. Moreover, the invention asdisclosed herein, may be suitably practiced in the absence of thespecific elements which are disclosed herein.

1. A tunable circuit for a capacitively tunable device, the tunablecircuit comprising: a substrate; a tunable circuit element; anon-tunable dielectric clement coupled to the tunable circuit element,the substrate, tunable circuit element, and non-tunable dielectricelement configured in a layered structure, wherein the tunable circuitelement and non-tunable dielectric element are configured in a layeredstructure with the tunable circuit element being layered upon thenon-tunable dielectric element and a pair of contacts contacting thetunable circuit element; and at least one AC terminal contacting onlythe non-tunable dielectric element.
 2. The tunable circuit of claim 1wherein the tunable circuit element and non-tunable dielectric elementare coupled together in series.
 3. The tunable circuit of claim 1wherein the tunable circuit element and non-tunable dielectric elementare configured as a single lumped element.
 4. The tunable circuit ofclaim 1 wherein the non-tunable dielectric element is selected from thegroup consisting of non-tunable oxides and dielectric polymers.
 5. Thetunable circuit of claim 1 wherein the non-tunable dielectric element isdeposited by a method selected from the group consisting of physicalvapor deposition, spin coating, and ink jet writing.
 6. A method forincreasing the figure of merit in a tunable capacitor device, the methodcomprising: providing a substrate; providing a tunable element;providing a non-tunable dielectric element with at least one AC terminalcontacting only the non-tunable dielectric clement; coupling the tunableelement to the non-tunable dielectric element so that the substrate,tunable element, and non-tunable dielectric element are provided in alayered structure; and configuring the tunable element and thenon-tunable dielectric element in a layered structure with the tunableelement being layered upon the non-tunable dielectric element and a pairof contacts contacting the tunable element.
 7. The method of claim 6 andfurther comprising: selecting the non-tunable dielectric element fromthe group consisting of non-tunable oxides and dielectric polymers. 8.The method of claim 6 and further comprising: depositing the non-tunabledielectric element by a method selected from the group consisting ofphysical vapor deposition, spin coating, and ink jet writing.
 9. Themethod of claim 6 and further comprising: coupling the tunable elementand the non-tunable dielectric element in series.
 10. The method ofclaim 8 and further comprising: configuring the tunable element and thenon-tunable dielectric element as a single lumped element.
 11. A tunablecapacitor device comprising: a substrate; non-tunable dielectricelement; a tunable circuit element electrically connected to thenon-tunable dielectric element forming a combined dielectric element,the combined dielectric element being electrically connected to thesubstrate, the substrate, non-tunable dielectric element, and tunablecircuit element configured in a layered structure with the tunablecircuit element being layered upon the non-tunable dielectric element;at least one AC terminal contacting only the non-tunable dielectricelement; and a plurality of contacts mounted to the tunable circuitelement.
 12. The tunable capacitor device of claim 11 wherein thetunable circuit element and the non-tunable dielectric element areconfigured as a single lumped element.
 13. The tunable capacitor deviceof claim 11 wherein the tunable circuit element and the non-tunabledielectric element are configured in a layered structure.
 14. Thetunable capacitor device of claim 11 wherein the non-tunable dielectricelement is selected from the group consisting of non-tunable oxides anddielectric polymers.
 15. The tunable capacitor device of claim 11wherein the non-tunable dielectric element is deposited by a methodselected from the group consisting of physical vapor deposition, spincoating, and ink jet writing.
 16. The tunable capacitor device of claim11 wherein the tunable circuit element directly contacts the non-tunabledielectric element.
 17. The tunable capacitor device of claim 16 whereineach contact electrically contacts one of the tunable circuit elementand the non-tunable dielectric element.
 18. The tunable capacitor deviceof claim 11 wherein the non-tunable dielectric element directly contactsthe substrate.
 19. The tunable capacitor device of claim 18 wherein atleast two contacts electrically contact the tunable circuit element andat least one contact electrically contacts the non-tunable dielectricelement.
 20. The tunable capacitor device of claim 11 wherein thenon-tunable dielectric element is electrically connected to the tunablecircuit element by a wire, solder, or other electrical contact.
 21. Thetunable capacitor device of claim 11 wherein the tunable circuit elementand the non-tunable dielectric element are coupled together in series.